Grease Pencil Onto video
Start with the template for 2d animation.

Set the color management to sRGB, the view transform to Standard and the look to None.

Press Tab to enter Edit mode.
In the outliner select the camera.

In the properties window select camera and enable Background Image.
Set the source to Movie Clip, select your file and set the opacity to 100%

This will allow you to use the video as background image when you are looking trough the camera view. (Note that this image is for reference only, and will not render without setting it up in the outliner)
Select stroke on the outliner (or create a new one using Add &gt; Grease Pencil &gt; Blank).
Enter draw mode and draw on top of the background video.

To render the grease pencil along with the video, set film &gt; Transparent

Then set the compositor with a movie clip node with the video and the render layer and use alpha over to composite them together.